It has been conventional in manufacturing various kinds of steel materials subjected to a brazing treatment that the steel material is heated to a temperature of about 1100.degree.-1200.degree. C. for effecting a copper solder brazing treatment, using a converted gas, decomposed ammonia gas or the like as the atmosphere in the furnace, and thereafter gradually cooling the steel in the furnace to room temperature as shown by furnace cooling curve a in FIG. 1.
The resultant heat-treated product is lower in mechanical properties, such as fatigue strength, bending strength as compared to the untreated steel material before the brazing treatment. Also, the product is comparatively weak in tensile strength at its soldered portion.
Therefore, in order to produce a product having the required mechanical properties after the heat treatment, it has been necessary for the product to have increased thickness to use a raw material having higher mechanical properties. This leads to the disadvantage of increase in weight of the product, higher cost, and the like.